Shri Indra was the favourite national god of the Vedic
Indians, with about 250 hymns dedicated to him in the Rigveda, more
than those devoted to any other god and almost one fourth of the total
number of hymns of the Rigveda.

Shri Indra is the god of thunder, being similar in many ways
to the Teutonic Thor (Old Norse Þorr; Old English Þunor; German Donner), or Greek Zeus/Roman Jupiter. Like Thor, he is
described in the Rigveda as red- or tawny-beared (RV 10.23,4), though
the extant sculpture and carvings seem to invariably show him as beardless.
His characteristic weapon is the cudgel vajra ('thunder-bolt'),
just as Thor's is the famous hammer Mjöllnir. Shri Indra is the
pre-eminent drinker of the divine soma (the madhu or 'mead'
of the gods), the imbibing of which exhilerates him and aids him in his
heroic actions.

Shri Indra's most important deed is the slaying of the asura
('demon') Vritra, who is a Dragon (ahi). Vritrahoards
and encloses the vital elements of the universe: waters, light, cows,
and Shri Indra struggled in order to free these life-sustaining treasures
from the grasp of the dragon Vritra(whose very name, Vṛtra,
means 'the Encloser, the Obstructer') and release them into the world.
And he must continually struggle against this Dragon (ahi), who represents
the forces of Chaos and Non-Existence (asat).This is why one of
Shri Indra's epithets is Vṛtrahan 'slayer of Vritra, overcomer
of resistance'. By defeating these forces he separates and supports heaven
and earth (RV 5.29). The Norse thunder-god Thor too battles against the
midgardsorm ('serpent of Middle-Earth') at various times, as do other
Indo-European Storm-Gods, such as the Hittite Tarhunnas (the weather-god)
against the dragon Illuyankas.

In his battle with Vritra and other demons, such as Namuci (na-muci
'he who won't release'), his especial friend is Shri Vishnu (RV
1.22,19), who is often described as helping Indra to defeat Vritra. Vishnu's
function in the battle with Vritra seems to be primarily to 'make space'
by means of his famous three strides, for Indra, about to slay Vritra,
says to Vishnu: 'O my friend Vishnu, stride out widely' (RV 4.18,11).
The association of Arjuna (son of Indra) and Shri Krishna (avatara of Vishnu),
especially in their companionship during the Mahabharata battle, is thus
prefigured by this association of Shri Indra and Vishnu.

By worship and sacrifice (especially soma-sacrifice, as soma-drinking increases Indra's strength and vigour), humans on earth help Indra in his struggles, e.g.:

'Sacrifice, Indra, made you grow so mighty, the dear oblation with the pressed soma.
O Worshipful, with sacrifice help our sacrifice, for sacrifice helped your vajra when slaying the dragon'.
(Rigveda 3.32,12; cf. RV i.63,2)

See further below on soma-sacrifice.
.......

Prayers to Indra

Rgveda I.32:

I now proclaim the heroic deeds of Indra; what he did first, the vajra-wielder,
he slew the dragon, bored through to the waters; he split through the bellies of the mountains. [1]

He slew the dragon who lay on the mountain; Tvashtar (the smith of the gods) fashioned for him the roaring Vajra (thunder-club). [With a sound] like lowing milch cows, the flowing waters ran quickly down to the ocean. [2]

Bullishly, he chose for himself the Soma (ambrosia of the gods); he drank from the pressed (soma) in the three soma-vessels. The Generous One (=Indra) took the missile, the vajra; he slew him, the first-born of dragons. [3]

When, you, O Indra, slew the first-born of dragons and thereupon dispelled the sorcery of the sorcerers, then producing the sun, the day, the dawn, you found no enemy indeed at that time. [4]

Indra slew Vritra (the dragon), the greatest of Vritras, with the vajra; (slew) the shoulderless one with his great weapon. Like branches/shoulders of a tree hewn off by an axe, the dragon lies flat on the ground. [5]

Like one ignorant of battle, badly intoxicated, (the dragon) challenged the advancing great warrior, the presser of many; (he) did not escape the collision of his weapons; he who had Indra as a conqueror crushed together the rivers (?). [6]

Footless and handless he fought Indra; (Indra) struck him on the ridge (=neck?) with his vajra. The gelded one who wished to be equal to the bull, the serpent lay cut into pieces strewn in many places. [7]

He (=the dragon) lay there like a split reed, the rising waters of Manu flowed over (him); the dragon lay at the feet of those whom he had encompassed with his might. [8]

Vritra's mother's strength became low; Indra warded off her weapon. The mother lay above, the son lay below; Danu (=Vritra's mother) lay like a milch cow with her calf. [9]

Vritra's body was sunk in the midst of the never standing, never resting water courses; the waters flowing over Vritra's secret, he who was conquered by Indra lay in the long darkness. [10]

The waters were as wives of the Dasa (=Vritra), guarded by the dragon, shut up like cows by the Pani. The orifice of the waters, which had been closed up, he (=Indra) opened after having slain the dragon. [11]

You became a tail-hair of a horse there, O Indra, when you struck against the missile; you, the unique god, conquered the cows; you conquered, O Hero, the Soma; you let flow free the seven rivers. [12]

Not for him [=Vritra] did the lightning avail, nor thunder, no what mist and hail-stones he scattered. When Indra and the dragon fought, the Generous One [=Indra] was victorious (then) and for the future. [13]

What avenger of the dragon did you see, O Indra, when fear came into the your heart, the heart of the one who slew the dragon, when you crossed nine and ninety streams; like a startled eagle crossed through the cloudy regions? [14]

Indra, the vajra-wielder, is king over that which moves and that which stays still, over the tamed and the horned; he rules as king over humans, containing (all) as spokes within the wheel's rim. [15]

Soma yajna
(sacrifice) / [Indrapuja]

More to be added...

Soma is to be identified with the plant Ephedra (vulgaris / distachya)
.............
Rigveda - Book 9, Hymn 4 (Griffith's translation; to be updated)
O flowing Soma, win and conquer high fame; [1ab]
And make us better than we are. [1cd]
Win the light, win heaven, and, Soma, all good things; [2ab]
And make us better than we are. [2cd]
Win skilful strength and mental power. O Soma, drive away our foes; [3ab]
And make us better than we are. [3cd]
Filterers- purify soma for Indra, for his drink: [4ab]
And make us better than we are. [4cd]
Give us our portion in the Sun through your own mental power and aids; [5ab]
And make us better than we are. [5cd]
Through your own mental power and aid long may we look upon the Sun; [6ab]
And make us better than we are. [6cd]
Well-weaponed Soma, pour to us stream of riches doubly great; [7ab]
And make us better than we are. [7cd]
As one victorious unsubdued in battle pour forth wealth to us; [8ab]
And make us better than we are. [8cd]
By worship, O flowing Soma, men have augmented you for support: [9ab]
And make us better than we are. [9cd]
O Indu ['soma drop']], bring us wealth in steeds, manifold, all-reaching; [10ab]
And make us better than we are. [10cd]